Log backtrace for the errors in actor
Add steps to the suspended_step_manager before they are run

This way, there is no chance some event would come sooner than the
step is propagated to the suspended_steps_manager
Add steps to the suspended_step_manager before they are run

This way, there is no chance some event would come sooner than the
step is propagated to the suspended_steps_manager
More descriptive string representation of step
Rename SuspendedStepsManager to RunningStepsManager

As it manages the steps even before they get into the suspended state.
Push nil instead of step into @events in RunningStepsManager

It makes more sense this way
Fix UnprocessableEvent parent
Add type-checks to WorkQueue, store Work::Step instead of nil